1. Field of the Invention
The present invention relates to a gain control circuit and particularly to a gain control circuit in a wide frequency band high-frequency amplifying circuit in which the amount of gain control can be made even over the whole frequency band with less non-linear distortion.
2. Description of Prior Art
FIG. 1 shows the conventional gain control circuit which comprises an input terminal 1, a transistor 2 for amplifying a high-frequency signal f.sub.o from the input terminal 1, and a variable impedance element 3 such as a PIN diode with one end connected with the emitter of the transistor 2 and the other end is grounded through a capacitor 4. In view of equivalence, the variable impedance circuit 3 is such that an internal resistor 3a and an internal capacitor 3b are connected in parallel with each other and the resistance value of the internal resistor 3a is varied in accordance with the amount of current i1 flowing thereinto to thereby change the impedance. The gain control circuit further comprises another transistor 5 for controlling the amount of current flowing in the variable impedance element 3, an output terminal 13 connected with the collector of the transistor 2, an input terminal 14 for receiving a gain control voltage which is applied to the base of the transistor 5 through a resistor 11 to control the base voltage which in turn controls the collector current so that the current flowing into the impedance element 3 is controlled, a power supply terminal to which a negative voltage is applied, resistors 6 to 10, and a coil 12.
The operation of the thus arranged gain control circuit will be described hereunder. The high-frequency signal f.sub.o applied to the input terminal is fed to the base of the transistor 2, amplified therein by means of amplifying function thereof, and produced to the next stage (not shown) through the output terminal 13. At this time, the emitter current i1 flows also through the transistor 5 and the collector current of the transistor 5 is controlled by the base voltage applied from the input terminal of the transistor 5. The variable impedance element 3, such as a PIN diode, is arranged such that the resistance value of the internal resistor 3a is varied in accordance with the amount of current flowing thereinto so that the collector current of the transistor 5 is controlled by controlling the base voltage thereof to control the amount of current flowing in the variable impedance element 3 to thereby vary the resistance value of the internal resistor 3a. As the amount of current flowing in the variable impedance element 3 increases, the resistance value of the internal resistor 3a and therefore the impedance of the variable impedance element 3 becomes low to increase the collector current of the transistor 2 to thereby make the signal amplifying gain of the transistor 2 high. The reverse applies the case where the resistance value of the internal resistor value 3a is made high, whereby the signal amplifying gain can be controlled.
In the thus arranged gain control circuit, for example, if it is desired to lower the gain, the base voltage of the transistor 5 applied from the input terminal 14 is lowered to decrease the collector current of the transistor 5. Thus, the amount of current flowing in the variable impedance element 3 which is equal to the collector current of the transistor 5 also decreases. As the amount of current of the variable impedance element 3 decreases, the impedance for the high-frequency signal f.sub.o increases correspondingly (see FIG. 3) and current negative feedback is applied to the transistor 2 due to the emitter impedance thereof so that the gain is lowered as shown in FIG. 4. In the case the current flowing in the variable impedance element 3 takes a minimum value, the gain of the transistor 2 becomes minimum (the amount of attenuation is the largest).
In the thus arranged gain control circuit as described above, however, the impedance (Z) of the variable impedance element 3 takes a different value depending on the frequency f.sub.o of the high-frequency signal as shown in FIG. 3 even if the amount of current flowing in the variable impedance element 3 is constant (for example I.sub.D =I.sub.I), so that the respective amounts of gain control G1 and G2 are different from each other (G.sub.1 &gt;G.sub.2) between high-frequency signals f.sub.1 and f.sub.2 (f.sub.1 &gt;f.sub.2) in the high and low frequency bands respectively as shown in FIG. 4, resulting in variations in amount of attenuation depending on the frequency of the high-frequency signal f.sub.o.
If the base voltage of the transistor 5 is lowered to lower the gain, the emitter current of the transistor 2 is caused to decrease, resulting in a change in the operating point of the transistor 2 to enlarge the non-linear distortion. Thus, the distortion characteristic of the amplifying circuit with the transistor 2 as its amplifying element is deteriorated.
Further, in the case where the operating point of the amplifying transistor 2 shifts due to the reduction in the emitter current thereof, the level of the high-frequency signal applied to the base of the transistor 2 is large as it was before the operating point has been shifted so that the high-frequency signal of such a large signal level can not be sufficiently amplified by the transistor 2 the emitter current of which has been decreased and this fact also may be a cause of deterioration in non-linear distortion characteristic.